Elastocaloric properties of thermoplastic polyurethane

Abstract

Very few studies have explored the elastocaloric effect of elastomers other than natural rubber (NR). The aim of the present article is thus to evaluate the elastocaloric properties of a thermoplastic polyurethane (TPU) in terms of microstructural characteristics and thermoelastic coupling. Calorimetric measurements showed two successive peaks at 240 K and 282 K, attributed to the crystallization and melting of soft segments, respectively. X-ray diffraction indicated that TPU exhibited a fully reversible strain-induced crystallization at room temperature. Thermomechanical experiments performed at different elongations revealed a minimum adiabatic temperature variation of about -8 K after retraction of a sample initially elongated at λ = 5. This is comparable to NR performances. However, for cycles carried out between λ = 1 and λ = 5, tensile stress/elongation curves showed a non-elastic behavior of TPU. A pseudo-elastic response was obtained for cyclic elongation when unloading was incomplete, in our case, when λ was between 3 and 5. The recorded peak-to-peak temperature variation decreased from 4.5 K to 3.3 K when the number of cycles was increased to 5000. Despite the fact that the issue of fatigue resistance for TPU needs to be addressed, this work opens new perspectives for studying the elastocaloric properties of various polyurethanes (whether crosslinked or thermoplastic) as well as other materials with a tendency for strain-induced crystallization, such as polychloroprene, hydrogenated acrylonitrile butadiene rubber, and others.